Normally, a "black label" 6V starter will have an armature diameter of about 5/8", and it will be matched up with a 139 tooth flywheel. The 1955-up "red label" starter will have a shaft diameter close to 1/2", and the correct flywheel will have 168 teeth. BOTH starter drives have 9 teeth, but the tooth pitch will be slightly different. It might be possible to mix and match parts to get a 6V housing and field coils combined with a 12V armature and drive gear, but I have my doubts that it would crank at the proper speed with a 6V battery.
Jerry

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